Increased polymerization reactor output by using a specific initiator system

ABSTRACT

The invention pertains to a process to polymerize one or more monomers wherein at most 90% w/w of a safely useable amount of a first initiator is used and a second initiator is dosed at least after the start of the polymerization, in an amount such that essentially the full cooling capacity of the polymerization reactor is used, resulting in a cost efficient process to make polymers, particularly polymers comprising polymerized vinyl chloride.

The present invention relates to a process to polymerize one or moremonomers in a reactor with a certain cooling capacity, wherein saidcooling capacity is limiting the space-time yield of the reactor.

DE-OS-1 570 963 discloses to dose an initiator, optionally mixed with asolvent, to the mixture to be polymerized via a stream of water. Theorganic peroxides that are exemplified in this patent application areperoxydicarbonates and acetyl-cyclohexyl-sulfonyl-peroxide (ACSP). Thisprocess of DE-OS-1 570 963 was found to solve a number of problems inthe industry. However, the process still suffers from insufficientcontrol of the heat peak of the polymerization reaction and a relatedless than optimal space-time yield of the reactor due to coolingcapacity restrictions. Accordingly, a different process not sufferingfrom these drawbacks is desired.

Similarly, EP-A-0 096 365 discloses how a peroxide is added in threeparts during the polymerization. Again, difficulties in controlling theheat that is subsequently generated are reported.

JP-A-07082304 discloses a process wherein a first peroxide is used atthe start of the polymerization. Later in the process, when a refluxcondenser is used as an additional cooling device, a second peroxide isdosed to the reactor to more efficiently use the additional coolingcapacity of the condenser. Only a limited amount of the second peroxidecan be used due to the fact that the first peroxide is still used in aconventional amount. By doing so the polymerization time is reduced from5 hours and 3 minutes for the conventional process, in which only thefirst peroxide is used, to 4 hours and 17 minutes for the processwherein additionally the second peroxide is dosed. Although asignificant improvement over the prior art, the time gain is stillconsidered less than desired. Even higher time-space yield gains arebeing looked for.

Furthermore, the above-mentioned processes of the prior art are not verysuitable for increasing the space-time yield of very largepolymerization reactors, particularly those of at least 15 m³, moreparticularly those of at least 20 m³, wherein safety margins aremaintained to prevent the polymerizing content from “running away”,meaning that due to excessive heat development the reactor content isheated to above its set temperature, causing more of the conventionalperoxide to decompose, which causes the polymerization rate to furtherincrease, thus auto-acceleration occurs up to the point that the reactorcontent has to be stopped or dumped or it will rupture the reactor.

The current invention relates to a new process wherein these problemshave largely been solved. More particularly, we have found that byselecting the proper amount of initiator that is dosed at the beginningof the polymerization and the proper dosing conditions for subsequentlyadded initiator, it is possible to obtain a polymerization reactionwhere the rate of polymerization can be very well controlled andtherefore the heat of polymerization can be kept at the maximum coolingcapacity, allowing optimum reactor space-time yield, very efficientperoxide usage, and very low residual peroxide levels in the resin afterpolymerization. Furthermore, the fact that less of a conventional firstinitiator is dosed at the start of the polymerization, it was found thatthe distribution of said first initiator over the monomer was improved.Especially for the suspension and emulsion polymerization process thiswas found to result in polymer particles with less defects, such asfish-eyes, that are considered to be caused by too high peroxideconcentration in a monomer droplet. This effect can be further augmentedby metering both the initiator and a protective colloid at the sametime. Additionally, the porosity of the polymer can be enhanced andbetter controlled through selection of the appropriate protectivecolloid and the way in which the protective colloid and/or the peroxideare metered.

The new process is characterized in that, compared to a process which isrun at its maximum rate with all initiator being added at the start ofthe polymerization, only up to 90 mole % of said initiator (or mixtureof initiators) is added at the start of the polymerization and a secondinitiator, which is less temperature stable than the first initiatorthat is added at the start, said second initiator having a half-lifefrom 0.0001 hour to 1.0 hour at the polymerization temperature, is dosedto the polymerizing mixture in an amount, and preferably at a varyingrate, such that at least 92, preferably at least 96, more preferably atleast 98% of the maximum cooling capacity is used during at least aperiod of time wherein at least 10% by weight (% w/w), preferably 20%w/w, more preferably at least 30% w/w, most preferably at least 40% w/wof the monomer is polymerized. Preferably the second initiator is dosedin such a way that the maximum cooling capacity is used in a period oftime wherein at least 10% by weight (% w/w), preferably 20% w/w, morepreferably at least 30% w/w, most preferably at least 40% w/w of themonomer is polymerized, whereby the actual temperature of thepolymerizing mixture is kept at the desired temperature plus or minus0.2° C., because the more the maximum cooling capacity is being used,the more efficient is the polymerization process. The dosing of thesecond initiator in conjunction with the use of the first initiatorallows for an unprecedented i) accurate control of the polymerizationrate and related polymerization heat generation (and, accordingly, alsothe polymerization temperature) by controlling the peroxide dosing rate,ii) space-time yield of the reactor, and iii) very economic initiatorusage.

If the amount of initiator being added at the start of thepolymerization is carefully chosen and the second initiator is dosedaccording to the invention, then it is furthermore found possible tosignificantly reduce or remove the run-away safety margins that werenecessary in the conventional process and the process can actually runat such a rate that the heat of polymerization is very close to, or itmay even temporarily exceed, the cooling capacity, without that such arun-away is observed. In this respect, reference is made toJP-A-07082304 (see Example 1) in which conventional safety margins arebeing maintained of more than 15% of the total cooling capacity. In aprocess according to the invention the actual temperature of thereacting mixture typically does not reach a temperature that is morethan 6° C. above the desired polymerization temperature. Preferably, theovershoot of the temperature of the polymerization mixture is less than4° C., more preferably it is less than 2° C., even more preferably it isless than 1° C. Most preferably, the actual temperature of thepolymerizing mixture is kept at the desired temperature plus or minus0.2° C.

The amount of initiator being added at the start of the polymerizationshould be at most 90% by weight (% w/w) of the maximum amount of saidinitiator that can be used in the identical process that is run at themaximum cooling capacity and whereof the temperature is not exceedingthe set temperature due to an excess of polymerization heat. If morethan this maximum amount of said initiator is used, the polymerizationheat exceeds the heat transfer capacity of the reactor and the reactorcontent will heat up to a temperature above the maximum allowablepolymerization temperature. Since such an overshoot in temperature willcause a more rapid decomposition of the initiator, the temperature mayeven increase further, up to a point where the reaction runs away andbecomes uncontrollable and hazardous. For this reason this maximumamount of initiator is called the safely useable amount. Preferably itis at most 80% w/w of said safely useable amount, more preferably atmost 70% w/w, and most preferably at most 65% w/w. The lower the amountof first initiator the better the controlling of the polymerization rate(and, accordingly, the heat of polymerization) during the process.Preferably at least 2% w/w, more preferably at least 5% w/w, mostpreferably at least 10% w/w of said safely useable amount of firstinitiator is used.

It is noted that the term “maximum cooling capacity” as used herein isused in its conventional meaning, being the amount of heat that can beremoved from the polymerization reaction mixture when 1) said reactionmixture has a temperature equal to the preset polymerizationtemperature, and 2) the cooling capacity is at its maximum level undernormal polymerization conditions (typically the situation wherein thecooling medium is flowing at its maximum rate at its lowest temperatureunder normal polymerization conditions).

The amount of the second initiator that is used is preferably at least0.01% by weight (% w/w), more preferably at least 0.015% w/w, and mostpreferably more than 0.02% w/w, all based on the weight of the monomerthat is polymerized in the process.

It is noted that WO 00/17245 discloses a process wherein initiators aredosed to a polymerization reactor at temperatures where essentially allthe initiators have a half-life of from 0.05 to 1.0 hour. Innon-prepublished application PCT/EP02/14518 extremely fast organicinitiators with a half-life from 0.0001 hour to 0.050 hour at thepolymerization temperature are used to give an improved control of thepolymerization rate, higher polymerization rates, leading to anincreased space-time yield of polymerization reactors, and results inthe process rendering a polymer with very low residual initiator levels.However, these processes were found to require the use of high amountsof the fast or extremely fast peroxides because the initiator efficiencyis below that of a conventional peroxide that is dosed at the start.With the present process, the amount of fast or extremely fast peroxidethat is used is lower than the amount as disclosed in WO 00/17245 andPCT/EP02/14518. Also the total amount of active oxygen as needed in thepresent process, compared to the process of WO 00/17245 andPCT/EP02/14518 with the same polymerization time, was found to bereduced. Therefore, the present process is more economical and yields apolymer with less decomposition products of the initiator, hence aproduct with improved organoleptic properties, particularly smell. Also,it is known that residual decomposition products typically having amolecular weight of less than 250 Dalton may lead to fogging (thedecomposition products evaporate from the resin and condense on anothersurface), which is undesired. Also, a rework of Example F of WO 00/17245showed that only after 2.9 hours after the start of the heat-up, andonly for an instant, the maximum cooling capacity was used. It wasfurthermore observed that the present process allows for a betterdistribution of the initiators over the monomer, which, particularlywhen the polymerization is a dispersion polymerization, such as anemulsion or suspension polymerization, results in a polymer withimproved properties. Particularly the molecular weight and/or themolecular weight distribution of the polymer and/or the number offish-eyes (resulting from the polymerization of a monomer droplet when atoo high peroxide concentration was present) was found to be improved.

The process according to the invention is preeminently suited topolymerize monomer mixtures comprising vinyl chloride monomer (VCM).Preferably, the process according to the invention involves thepolymerization of monomer mixtures comprising at least 50% by weight (%w/w) of VCM, based on the weight of all monomer. Comonomers that can beused are of the conventional type and include vinylidene chloride, vinylacetate, ethylene, propylene, acrylonitrile, styrene, and(meth)acrylates. More preferably, at least 80% w/w of the monomer(s)being polymerized is made up of VCM, while in the most preferred processthe monomer consists essentially of VCM. As is known in the art, thepolymerization temperature of such processes to a large extentdetermines the molecular weight of the resulting resin.

It is to be understood that the word “dosing” is used to describe thestep of adding initiator to the polymerizing reaction mixture atpolymerization conditions. The dosing can be done intermittently duringthe polymerization, meaning that at least two portions of initiator areadded to the reaction mixture, or it can be continuous, meaning that fora certain period of time the initiator is continuously added to thereaction mixture, or any combination of these techniques. Examples of acombination of such techniques include, for instance, a process whereinthe initiator is first added continuously, then the addition is stopped,and then again it is added continuously. If an intermittent operation isselected, there are at least 2, preferably at least 4, more preferablyat least 10, and most preferably at least 20 moments at thepolymerization temperature at which the initiator is dosed. Mostpreferably, the peroxide is dosed continuously and/or intermittentlyfrom the start of the polymerization reaction, preferably after at least5%, less preferred after at least 10%, even more less preferred after atleast 20%, of the monomer(s) has already been polymerized and whereinduring the dosing period at least 2, preferably at least 5, morepreferably at least 10%, more preferably at least 20%, more preferablyat least 40%, and most preferably at least 60%, of all monomer used inthe process is polymerized.

In the process according to the invention, one or more initiators may beused as the second initiator, preferably the initiators are selectedfrom organic peroxides, however, they may also be selected fromconventional azo-initiators. Preferred examples of peroxides to be usedas a second initiator in the process according to the invention are thefollowing:

-   -   1,1,3,3-tetramethylbutylperoxy methoxy acetate, or hexanoyl        pivaloyl peroxide for polymerization reactions at 35-70° C.,        preferably 40-65° C.    -   diisobutanoylperoxide, bis(tert-butylperoxy) oxalate or        2,2-bis(2,2-dimethylpropanoylperoxy)-4-methyl pentane, for        polymerization reactions at 40-85° C., preferably 45-80° C.    -   α-cumyl peroxyneodecanoate,        2-(2,2-dimethylpropanoylperoxy)-2-(2-ethylhexanoylperoxy)-4-methyl        pentane or 2,4,4-trimethylpentyl-2-peroxyneodecanoate, at        polymerization temperatures of 53-99° C., preferably 60-95° C.,        and    -   tert-amyl, tert-butyl peroxyneodecanoate or peroxydicarbonates,        at polymerization temperatures of 62-107° C., preferably 75-100°        C.

Other peroxides may also be used. Their half-lives can be determined byconventional thermal decomposition studies in monochlorobenzene, aswell-known in the art (see for instance the brochure “Initiators forhigh polymers” with code 10737 obtainable from Akzo Nobel). The termpolymerization temperature as used herein is used in its conventionalconnotation and represent the average temperature in the period in whichthe majority of the monomer is polymerized. In case of doubt, it is theaverage temperature in the polymerization period in which not the firstand not the last 5% of the monomer is polymerized.

Preferably, the first initiator that is added at the start of thepolymerization, meaning when up to 10% of the monomer has polymerized,preferably when up to 5% of the monomer has polymerized, more preferablywhen up to 2% of the monomer has polymerized, even more preferably up to1% has polymerized, most preferably when essentially no monomer has yetpolymerized, has a half-life (when measured in monochlorobenzene at thepolymerization temperature) of from 0.1 hour to 10.0 hour. Morepreferably, essentially all peroxide that is added at the start has ahalf-life of 0.2 to 5.0 hour, even more preferably 0.4 to 2.0 hour, mostpreferably 0.5 to 1.0 hour. The first initiator can be a mixture ofinitiators, provided that the weight average half life of the initiatorsin the mixture is within the 0.1-10 hour range. As said, the firstinitiator should have a longer half-life than the second initiator (atthe polymerization temperature). Preferred first initiators includeα-cumyl peroxyneodecanoate,2-(2,2-dimethylpropanoylperoxy)-2-(2-ethylhexanoylperoxy)-4-methylpentane, 2,4,4-trimethylpentyl-2-peroxyneodecanoate,3-hydroxy-1,1-dimethylbutyl peroxyneodecanoate, tert-amylperoxyneodecanoate, tert-butyl peroxyneodecanoate, peroxydicarbonates(e.g. di-(2-ethylhexyl) peroxydicarbonate and di-(secbutyl)peroxydicarbonate), tert butyl peroxyneoheptanoate, tert butylperoxy pivalate, tert amyl peroyxpivalate, and dilauroyl peroxide.

It is noted that when the term “polymerization temperature” is used,that this is the temperature at which the majority of all monomer (i.e.more than 50% w/w, preferably more than 60% w/w, most preferably morethan 75% w/w of the monomer being polymerized) is being polymerized. Itis known that the polymerization temperature set-point can be variedover time. Known polymerization temperature variations forpolymerizations of vinyl chloride include an intentional highertemperature when the polymerization is started and/or a highertemperature upon pressured drop, both used to increase the reactoroutput. If a variation in the polymerization temperature is applied,then the polymerization temperature is considered to be the averagetemperature over time from the moment the polymerization temperature isreached until the pressure drop. It is noted that also in the process ofthe present invention, the polymerization temperature set-point duringthe start-up and pressure drop stages may be higher than the averagepolymerization temperature set-point.

Preferably, the second initiator that is dosed during the polymerizationhas a half-life (when measured in monochlorobenzene at thepolymerization temperature) of from 0.0001 hour to 1.0 hour. Morepreferably, essentially all second initiator that is used has ahalf-life of 0.0005 to 0.8 hour, even more preferably 0.001 to 0.5 hour,most preferably 0.005 to 0.35 hour. The second initiator can be a redoxinitiation system. In such a case the reducing agent, the oxidizingagent, or both can be dosed in accordance with the invention. For suchredox systems, the half-life of the redox system is the half-life asmeasured when all components of the system are present. However, in viewof the fact that redox systems typically contain heavy metals and/orundesired reducing agents, the initiators of the present inventionpreferably are not such redox initiation systems. The second initiatormay be a single initiator or a mixture of several initiators. If amixture is used, all initiators of said mixture should fulfill thehalf-life requirement. If a mixture of second initiators is used, thenpreferably all initiator of the mixture is less temperature stable thanthe first initiator. If the first initiator is a mixture of initiators,then it is preferred that the second initiator is less temperaturestable than all initiators in said mixture. If both the first and secondinitiator consists of a mixture of initiators, then it is preferred thatthe most thermally stable initiator of the second mixture is less stablethan the least stable initiator of the first mixture.

In a preferred embodiment, the invention relates to a process whereinthe polymerization mixture is formulated at a temperature below thereaction (polymerization) temperature and subsequently heated to reachsaid desired reaction temperature. In such a cold-start process thefirst initiator is added at the start as defined above. However, in sucha process is can be beneficial to add also some of the more reactivesecond initiator during the heat-up phase, since it boost the heat up ofthe reaction mixture. Preferably, from 0-60% by weight (% w/w), morepreferably 5-40% w/w, most preferably 5-20% w/w, of the peroxide, basedon the total weight of the second initiator used during thepolymerization, is present at the start of the heating-up phase, whilethe remainder is dosed over a period of at least 1, preferably 2, andmore preferably 24 hours, depending on the polymerization time. Morepreferably, the remainder of the peroxide is dosed from the time thereaction mixture temperature is controlled at the desired reactiontemperature. The use of a combination of the first initiator and a smallamount of the second initiator from the start allows a fast heating upand start of the polymerization, since these initiators will already(partly) decompose during the heating of the polymerization mixture.When the polymerization mixture reaches the polymerization temperature,the remainder of the peroxide can be dosed to the mixture to control thefurther polymerization rate. Preferably, the dosing is continuous,preferably at a variable rate, since this allows the most accuratecontrol of the polymerization rate and a constant polymerization heatoutput. The combination of the first and second initiator ensures thehighest initiator efficiency and space-time reactor yield. This isparticularly important for commercial reactors. Therefore thepolymerization process according to the invention is preferably carriedout in reactors of 15 m³ or more.

In another preferred embodiment, the reaction mixture is formulated ator near the polymerization temperature. In this process, hereinaftercalled warm-start process, it is not necessary to add a certain amountof the second initiator at the start while the remainder is dosed overtime. However, also in this warm-start process it can be beneficial toadd up to 30% w/w, preferably up to 20% w/w, most preferably up to 10%w/w, of the second initiator immediately after formation of the reactionmixture, the remainder being dosed over time. If this procedure is used,the second initiator preferably is added as the last ingredient. Thisprocedure is particularly preferred if a certain amount ofpolymerization inhibitor (a radical trapping species) is present in thereaction mixture. If such a radical scavenger is present, for instancebecause it is introduced with the monomer wherein it is typically usedas a stabilizer, the initially dosed peroxide will react with saidscavenger, thus preventing a delayed start of the polymerizationreaction.

It is furthermore preferred that two steps are present in the processafter the polymerization temperature is reached. In a first step with aduration of up to 90 minutes, preferably up to 60 minutes, and morepreferably up to 45 minutes, and with a minimum duration of 1 minute,preferably of 5 minutes, more preferably of 10 minutes, from 1 to 60%w/w, preferably from 5 to 40% w/w, of all second initiator is dosed,such that the desired cooling capacity, preferably the maximum one, morepreferably the maximum cooling capacity that is acceptable from a safetypoint of view, is reached at the end of this dosing step. Then, in asecond step, the remaining initiator is dosed (added over time) in anamount and at a rate to control the polymerization in such a way that atleast 92, preferably at least 96, more preferably at least 98% of themaximum cooling capacity is used during at least a period of timewherein at least 10% by weight (% w/w), preferably 20% w/w, morepreferably at least 30% w/w, most preferably at least 40% w/w of themonomer is polymerized. Typically, this means that a variable dosingrate is used in the second step.

The total amount of first and second initiator to be used in a processaccording to the invention is within the range conventionally used inpolymerization processes. Typically, from 0.01 to 1% w/w of initiator,more specifically 0.01-0.5% w/w, based on the weight of the monomer(s)to be polymerized, is used.

The part of the second initiator that is dosed to the reactor can be inthe pure form or, preferably, in the form of a dilute solution ordispersion (such as a suspension or emulsion). One or more suitablesolvents can be used to dilute the initiator. Preferably, such solventsare easily removed during the steps working up the polymer after thepolymerization process (such as alcohols), or they are of such a naturethat it is acceptable to leave them as a residue in the final polymer.Furthermore, it can be advantageous, but is not necessarily required,that such solvents do not adversely affect the thermal stability of theinitiator dissolved therein, as can be verified by analyzing thehalf-life temperature of the initiator in said solvent. An example of asuitable solvent is isododecane. If an initiator dispersion is dosed,then the dispersion can be of either the initiator itself or of asolution of the initiator, preferably in said suitable solvents.Preferably, the dispersion is an aqueous dispersion. Preferably, theinitiator is dosed in a concentration of 0.1 to 60% w/w, more preferably0.5 to 25% w/w, and most preferably 2 to 15% w/w. The more diluteinitiator solutions or dispersions ensure rapid mixing of the peroxideand the polymerization mixture, which leads to a more efficient use ofthe peroxide.

It can be beneficial to dose the second initiator together with aprotective colloid.

To improve the time-space yield of the polymerization process, it isadvantageous to dose at least part of the second initiator during thephase of the process after the start of the pressure drop and/or duringthe pressure drop, wherein the pressure drops due to depletion of themonomer. By the term “after the start of the pressure drop and/or duringthe pressure drop” is meant the time during which the pressure in thepolymerization reactor drops, including the 30 minutes, preferably 20minutes, more preferably 10 minutes, and most preferably 5 minutes,before the pressure drop is actually observed. Typically, the pressuredrop is said to have occurred when the pressure is 0.2 bar, preferably0.1 bar, lower than the (linearly extrapolated) pressure during theearlier part of the polymerization. The second initiator being addedafter the start of the pressure drop and/or during the pressure droppreferably has a half-life of less than 1 hour at the polymerizationtemperature, since than a relatively small residual amount of theinitiator will remain in the polymer formed. In order to reduce theresidual amount even more, it is preferred to add extremely fast secondinitiators having a half-life of less than 0.05 hour at thepolymerization temperature. However, a slower initiator can also beemployed. In that case it can be preferred to add a scavenger which isable to neutralize or destroy the residual initiator in any subsequentstep to such an extent that the residual amount of the initiator in thepolymer is acceptable. It is also contemplated to add a scavenger whenfast and/or extremely fast peroxides are used.

Preferably, the dosing can be effected at any suitable entry point tothe reactor. If water is added in the course of the polymerizationprocess, for example to compensate for the shrinkage of the reactorcontent due to the polymerization reaction, it can be advantageous touse the line through which this water is dosed to also dose theinitiator. It is noted that if the formation of the initiator is fastenough, one can dose the raw materials for said initiator into piping orhold-up vessels, from which the initiator is then fed into thepolymerization mixture. Alternatively, but less desired, there is theprocess wherein the raw materials are added to the polymerizationmixture. In all instances it can be beneficial to add stirring equipmentand/or heat exchangers to the feed lines in order to optimizeefficiency.

It is noted that, due to the use of the more stable first initiator, itis not preferred to conduct the present process such that 80% or more ofall monomer is polymerized within a period of 2 hours. With such shortpolymerization times the amount of residual first initiator would be toohigh.

The polymerization process can be conducted either as a mass processwherein the reaction mixture is predominantly monomer or as a suspensionprocess wherein the reaction mixture typically is a suspension ofmonomer in water, or as an emulsion or micro-emulsion process whereinthe monomer typically is emulsified in water. Preferably, the presentprocess is a mass, suspension or emulsion process. More preferably it isa suspension polymerization process. Most preferably it is a batchsuspension process. In these processes the usual additives will have tobe used. For example, if the monomer is present in the form of asuspension in water, the usual additives like surfactant(s), protectivecolloid(s), anti-fouling agent(s), pH-buffer(s), etc. may be present.Depending on the type of polymer desired, each of the above-mentionedprocesses may be preferred.

One or more protective colloids can be used in the process of theinvention. Examples of suitable protective colloids are protectivecolloids such as polyvinyl alcohols (PVAs), which may, for example, be(partially) saponified polyvinyl acetates with a degree of hydrolysis ofat least 40%, more preferably 60%, and most preferably 62%, and a degreeof hydrolysis of at most 90%, more preferably 85%, and most preferably80%. If for example two PVAs are employed, both PVAs may have a similardegree of hydrolysis. It may also be envisaged that the two PVAs have adifferent degree of hydrolysis. A first PVA may have a degree ofhydrolysis as described above. A second PVA may have a degree ofhydrolysis of at least 10%, more preferably 20%, and most preferably30%, and a degree of hydrolysis of at most 80%, more preferably 70%, andmost preferably 60%. If more than one PVA is used, the indicated degreeof hydrolysis generally is the weight-averaged degree of hydrolysis ofthe products used. Although said PVAs are the preferred protectivecolloids for processes according to the invention, it is also possibleto use other conventional protective colloids, such as cellulosics,water-soluble polymers, oil-soluble emulsifying agents or water-solubleemulsifying agents. Examples of such cellulosics are methyl cellulose,ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, andhydroxypropyl methyl cellulose. Examples of water-soluble polymers arepolyacrylic acid, gelatin, styrene maleic acid copolymers, and polyvinylpyrrolidone. Oil-soluble emulsifying agents are, for example, sorbitanmonolaurate, sorbitan trioleate, sorbitan monostearate, glycerintristearate, and ethylene oxide-propylene oxide block copolymers.Examples of water-soluble emulsifying agents are polyoxyethylenesorbitan monolaurate, polyoxyethylene glycerin oleate, and sodiumlaurate. It is also envisaged to employ a combination of two or more ofthe above protective colloids.

The protective colloid can be in a pure form or be diluted in anappropriate solvent, which in the case of PVA preferably is water or amixture of water and an alcohol. Aqueous solutions may contain at least0.05% PVA by weight, more preferably at least 0.5% by weight, and mostpreferably at least 1% by weight, and at most 40% PVA by weight, morepreferably at most 20% by weight, and most preferably at most 10% byweight. If so desired, the amount of PVA in the solution that isactually mixed into the reaction mixture can be even lower, for instancewhen an aqueous protective colloid solution as presented above is fedinto the reactor together with additional water that is fed to thereaction mixture.

The amount of protective colloid to be used in the process according tothe invention is within the lower ranges as conventionally used inpolymerization processes. Typically, this range has a lower limit of0.01% w/w of protective colloid and more preferably 0.02% w/w, and anupper limit of 1% w/w of protective colloid, preferably 0.3% w/w, andmost preferably 0.15% w/w, based on the weight of the monomer(s) to bepolymerized, is used. When it is said that protective colloid andinitiator are added at the same time, it is meant to include not onlyprocesses wherein initiator and protective colloid are addedsimultaneously or together, but also processes wherein initiator andprotective colloid are added or dosed in an alternating way orsequentially in random order at the polymerization temperature (eachbeing dosed at least twice). Also it is encompassed in the process ofthe present invention to dose at least part of the colloid just prior tothe pressure drop.

After the polymerization, the resulting (co)polymer (or resin) will beworked up as is usual in the art. Polymers obtained by a suspensionpolymerization according to the invention, for example, will besubmitted to the usual drying and screening steps. The resulting resinpreferably is characterized in that it contains less than 50 ppm ofresidual peroxide, more preferably less than 40 ppm, and mostpreferably, less than 25 ppm of peroxide, immediately after drying for 1hour at 60° C. and screening. The resin was found to exhibit excellentheat stability as measured with a Metrastat® PSD260 testing ovenaccording to method ISO 182-2 (1990E). The improved heat stabilityproved that the resin hardly discoloured when submitted tomelt-processing steps, e.g., to form shaped articles.

EXPERIMENTAL

Experiments are carried out according a standard suspensionpolymerisation process, using a 10 l Buchi reactor provided with onebaffle; three flat-bladed stirrers at three levels, a pressuretransducer, a vinyl chloride (VCM) feed line, a nitrogen purge line, aperoxide feed line and a peroxide injection point. The reactor ischarged with 4700 g of demineralized water; 40.2 g of a 5% solution ofAlcotex® B72 (polyvinyl acetate/alcohol) in demineralized water; andpressurized with nitrogen to 15 bars. If no leaks are observed, thereactor is evacuated for 5 min at 75 mbars (while stirred) to remove theair, and subsequently charged with 2870 g of VCM (ex Akzo Nobel Salt &Base), followed by heating up the reactor to the desired polymerisationtemperature of 57° C. in 30-60 minutes. After reaching a stabletemperature, the initial peroxide is dosed either by the injecting pointwithin 1 minute, the peroxide feed line during the polymerisationprocess or both, whatever is required. Dosing of the (very) fastperoxide through the feed line was done in such a way that the maximumrate (i.e. 33%/h) was reached in 0.5-0.65 hours. The cooling capacitywas limited to a value corresponding a maximum polymerization rate ofabout 33%/h (the maximum of the reference experiment). Thepolymerisation was stopped when a pressure drop of 2 bars was reached,by cooling the reactor and degassing the reactor. After removal of theresidual VCM by evacuation, the polymer was obtained by filtration,washing and drying. When an aqueous dispersion of the second initiatoris dosed during the process, the expected volume dosed is subtractedfrom the amount of water added in the beginning, so the total amount ofwater will be (more or less) the same at the end of the reaction.

Experiments 1 and 2 and Comparative Example A and B

Using the experimental set-up as described, a conventional initiator(Trigonox® EHP ex Akzo Nobel) was used as the first initiator. InComparative Example A this initiator was used as the sole initiator andit was found that the use of 656 ppm was the maximum amount that couldbe used whereby the reaction mixture maintained the desired reactiontemperature of 57° C. In Comparative Example B this maximum amount ofthe initial initiator was combined with an additional amount of thesecond initiator.

In Examples 1 and 2, less than the maximum amount of the first initiatorwas used in combination with an aqueous dispersion of a secondinitiator, Trigonox® 187 ex Akzo Nobel. The difference of the maximumtemperature that was observed and the preset temperature of 57° C. wasreported in the table as T incr.

The amount of polymer as obtained is presented as the yield on monomer(yield). The table furthermore presents some properties of the resultingpolymer. Psd is the average polymer particle size, and DOP is a measurefor the porosity of the polymer. The time till pressure drop (CPT) aswell as the time till the pressure had dropped to a value two bars belowthe pressure at the start of the pressure drop is also reported as ameasure of the polymerization rate.

Tx Time to EHP T incr Yield Psd DOP CPT −2 bar Experiment (ppm) Tx 187(ppm) (° C.) (%) (μm) (%) (min) (min) A 656 0 0 83.4 155.3 25.9 197 226B 656 310 6.0 89.1 154.8 24.8 139 170 1 500 410 0.8 86 153.6 25.5 146177 2 400 450 0 86.3 161.1 25.5 154 184

From these results it follows that much faster polymerization rates areattainable without that a run away of the reaction mixture temperatureis observed, when compared to conventional processes.

The PVC resulting from Experiments 1 and 2 had good organolepticproperties.

1. A process to polymerize one or more monomers wherein at most 90percent by weight of the safely useable amount of a first initiator isused and a second initiator, having a half-life from 0.0001 hour to 1.0hour at the polymerization temperature and that is less temperaturestable than said first initiator, is being dosed at least partially fromthe start of the polymerization until 10% of the monomer(s) has beenpolymerized, in an amount such that at least 92% of the maximum coolingcapacity is used during at least a period of time wherein at least 10percent by weight of the monomer is polymerized.
 2. A process accordingto claim 1 wherein the monomers comprise vinyl chloride.
 3. A processaccording to claim 2 wherein the process is a suspension polymerizationprocess.
 4. A process according to claim 1 wherein the second initiatoris additionally added intermittently and/or continuously after the startof the pressure drop and/or during the pressure drop.
 5. A processaccording to claim 1 wherein a protective colloid is added during thepolymerization process.
 6. A process according to claim 1 wherein saidfirst initiator has a half-life of 0.1 hour to 10 hours at thepolymerization temperature and the less temperature stable initiator hasa half-life of 0.0001 hour to 1.0 hour at said temperature.
 7. A processaccording to claim 1 wherein the amount of the second initiator that isused is at least 0.01% by weight, based on the weight of the monomerthat is polymerized.
 8. A process according to claim 1 wherein the totalamount of first and second initiator is 0.01 to 1% w/w, based on theweight of the monomer that is polymerized.
 9. A process according toclaim 4 wherein the polymerization reactor has a volume of 15 m³ ormore.
 10. A process according to claim 1 wherein said dosing of thefirst initiator is at a variable rate.
 11. A process according to claim3 wherein said suspension polymerization process is a batch suspensionpolymerization process showing a pressure drop of the vinyl chloride inthe reactor.
 12. A process according to claim 6 wherein a protectivecolloid is added during the polymerization process.